Electro-optical apparatus



- March 3, 1942 Filed Feb. 10, 1952 FIG? FIG. 3

A TTOR/VEY FIG. 4

FIG-5 60 W F IG. 0

l/'"82 aflg u 8.3 g1 857E 84 q 80- Ql 0 i as INVENTOR- hi E. IVESPatented Mar. 3, 1942 UNITED STATES ELECTRo-oPTIoAL'APfARArus Herbert E.Ives, Montclair, N. J., assignor to Bell Telephone Laboratories,Incorporated, New York, N. Y;, a corporation of New York ApplicationFebruary 10, issajseriai No. 591,966

(Cl.1786) I 'In the illustrative embodiment of thinv'ention herein shownand described, a two-tonePicture. is moved with respect to afixed'lightbeam at a transmitting station in such av manner that the.elemental areas of the picture'are illuminated v I in succession. Lightreflected fromthesucces- This invention relates to electro-opticalapparatus and particularly to an arrangement which is specially adaptedfor use in the reproduction of twof-tone pictures or messages, at asignal receiving station. r

One object of the invention is to provide a novel system fortransforming electrical variations into light variations. I

Another object of the invention is to provide an improved system forproducing images under 10 control of current transmitted over analternating current transmission line. 7 Another object is to increasethe speed of reproduction of a picture.

Another object is toautomatically correct for the distortion whichnormally results in a twotone reproduction of a picture due to thesuppression of low frequency components of the. picture current. V r

It is knownthat'a current produced as the result of scanning apicturehas a direct componentand alternating components extending over a rangeof frequencies and that, when such a picture current is directlytransmitted over. a

commercial telephone transmission line,'the direct component and some ofthe low frequency alternating components are suppressed .and the higherfrequency components only are transmitted- To avoid, the distortion inthe reproduced picture which would normally result dueto the suppressionof the direct and low frequency components, it has heretoforebeen thepractice to transmit a carrier wave modulated by the picture current..This practice has the disadvantage that it requires the transmission ofcurrent of a wide frequency range as compared with the frequencyrangerequired for transmit-5' ting the picture current without modulating acarrier wave. Since a given transmission line is capable of transmittingcurrent of a limited '40 ture currentlimits the speed of transmission ofa picture.

This difficulty is obviated,.in accordance with the present invention,by producing. an image under control of only the higher frequencycomponents of an image current which canbe transmitted over a commercialtransmission .line and automatically correcting for the distortion whichwould normally result due to the suppression of the lower. frequencycomponents ofthe image current.

si vely illuminated elemental area's impinges upon a light sensitivedevice to control the production of a picture current having'a .directcomponent and alternating componentsjextending over a' rangeoifrequencies' A transmission line which suppresses the direct andcertain low frequency componentsof the picture current and transmits thehigher frequency components thereof connects the transmitting stationwith a receiving station. A steady component, preferably correa spendingto the average'tone'value of the origv inal picture, is reinserted atthe receiving station.

This may be accomplished, for example, by impressing an alternating"electrom'otive force which varies in accordance with the transmittedpicture current upon the input circuit of a three electrode vacuum tubeto controlthe production of a unidirectional current, having alternatingcomponents corresponding to the components "of the transmitted picturecurrent, in 'its'output This unidirectional; current actuat'es a devicesuch as a' light valve for controlling the exposure of a moving lightsensitive surface for circuit.

controlling the reproduction of thepicture Due to the suppressionof thelow frequencycomponents of the picture current, there wouldordinarilyresult fluctuations in tone value of the reproduced picture which arenot present in-the original and which may become apparent in thereproduced picture ,as a mottled background for. example. In order toobviate distortion of this;

type, there is employed alight valve which emits no light when actuatedand which emits a: fixed I amount of light when unactuated, or viceversa';

regardless of the amplitude 'of the actuating -cur-.-'

rent. Theremay be employed, for example, a

light valve similar to that disclosed in Patent 1,667,805, granted to meMay; 1, .1928, but having a very narrow lightaperture so thattheaperture is completely covered for a certain range. I ofmovement ofthe string of the light valve and completely uncovered for a differentrange of movement of the light valve .string. 7 Referring to thedrawing: L-

mission system embodying ,the present, inven" tion; 1

Figs. 2 and 3 are detailed jviews of the "*valve employed in thesystemshown injig glf PATENT; OFFICE.

Fig. 1 shows schematically a picture transshowing the relationshipbetween the light valve aperture and the vibrating string for differentoperating conditions, respectively;

Figs. 4 to '7 ,are diagrams for facilitating the explanation of theoperation of the picture transmission system.

Referring to Fig. 1, a positive black and white picture Ill-which may bea typewritten message.

for example, is mountedon the drum II. The drum I l is rotated and atthe same time moved in an axial direction in a well known manner so thata beam of light from source l2 in effect traces a helical path on thepicture surface to illuminate the elemental areas thereof in succession.Light reflected from the successively vacuum tube amplifiers l4 and I5,transformers l6 and I! and the line l8 which connects the transmittingand receiving stations.

At thereceiving station a sheet of light sensitive photographic paper I9is mounted on a drum 20 similar to drum III at the transmitting stationand moved in synchronism therewith. The two drums may be maintained insynchroni'sm by the use of any of the well known methods employed for.thispurpose. For example, independent constant frequency currentsources may be employed forcontrolling the energization of the drivingmotors at the two stations as disclosed in Patent 1,808,923 granted toJ. .W. Horton June 9, 1931. The image current from amplifier lis'impressed upon the string 2! of the light valve 2| either throughtransformer 22 and a suitable biasing battery 23 or if desired,.thelight valvemay be connected directly in the anode circuit of the laststage of the vacuum tube amplifier I5.

The light valve 2| andthe optical system employed therewith may be ofthe type disclosed in Patent 1,667,805 granted to me May 1, 1928, exceptthat the light aperture of the'valve is very narrowwith respectto thewidth of the string 21 r and with respect to its amplitude .of movementso'that the light valve transmits a fixed amount of light when theamplitude ofthe energizing current is less than a fixed amplitude andtransmits no light when the amplitude of the energizing current isgreater than this fixed amplitude. The light valve comprises amagnetizing winding 24 which is mounted on a hollow central 'core' ofthe magnetic frame 25. A plate 26 is fastened to'the' frame 25 andprovides: an air chamber aperture 28 as shown in Fig. 2. When theamplitude is greater than this predetermined fixed value the aperture iscompletely covered by the string 21 as shown in Fig. 3;

Light from source 29 isdirectedup'on the light valve 2| by means of alens 30 so as to intensely and uniformly illuminate the aperture 28 ofthe light valve. A'1ens3l is arrang d sofas tdc'ause an image of thelight valve aperture to be formed upon the sheet of photographic paperl9.

It is now well known that the current produced in'the circuit of aphotoelectric cell as the result of scanning a picture may be analyzed,in accordance with a well known mathematical principle discovered byFourier, into "a steady direct component and a number of sinusoidal;components of different frequencies. The direct component isrepresentative of the average tone value of the entire picture. If weconsider the picture to be dividedinto several areas, the average tonevalue of one area may be different from the average tone value ofanother area, that is, one ,portion of the picture may be relativelydark. in

average tone value while another portion is relatively light. In aprinted page, for example, a

large number of letters may appear within a certain fixed area,resulting in a darkaverage tone value, while in another area of the samesize a much smaller number of letters may appear, thus resulting in alight average tone value. The relatively low frequencyalternatingcomponents of the image current represent-these changes in average tonevalue which occur in going from one portion of the picture to another.

The operation of'the system may be readily understood by considering thediagrams of Figs. 4 to 7. The area 50 represents a portion of thepicture having a black line 5| on 'a white ground 52, and the area 53represents a portion of the picture having a white line 54 one. blackground area 53 being dark, the average value of the currentrepresented'b'y curve-6| is'low'as indicated by the dotted 'line 64. Theareas 50 and 53 may represent extreme conditions which would piobablynot occur in practice, that is, the average tone value of differentportions of a picture would probably'not vary as much as indicated bythe dotted lines 63 and 64. As stated heretofore, ,a commercialtelephone transmission line including elements suchas the transformersi5 and I! will suppress the direct and certain low frequency alternatingcomponents. Moreover, the frequency range which can be transmittedsubstantially without distortion over such a signal transmission channelis known to be limited. Therefore, if a considerable band of the' lowfrequency components of the image current is'suppressed by asuitable'filterat the transmitting station, f

the upper limit of the frequency range which can be transmitted iscorrespondingly increased,'thus permitting an increase in thespee d ofscanning. I

Since the direct current component represented by the dotted lines 63and 64 of the current represented by curves 60 and SI is nottransmitted,

the received voltage wave impressed upon the amplifier l5 at thereceiving station or upon the secondary winding ofthe transformer 22 maybe represented by the'curves l0 andll, portions of which extend'aboveand'below the line of zero voltage" represented by thedotted line 12.Due

to the action'o'fthe battery 23 connected inseries with the secondarywinding of transformer 22 and the string-21 of the'light valve 2|, 'the'current flowingthrough the string of the light valve varies betweenzero, corresponding'to the dark element i .on the light ground 52, and

maximum amplitude corresponding to'the, light element 54 on the darkground 55 :as shown by the curves 8!] and 8|. It is apparentthat ifan.

ordinary type of light valve were to be employed to control the exposureof a negative film'from which a positive picture is produced, such apic-,

light gray, the amplitude 84 as darkgray, and.

the amplitude 85 as black. Thus, when trans-- mitting printed matterconsisting of black letters on a white ground, the letters inthereproducticn would appear as black butthe;background would appearmottled. When employing.

for thegtransmission of blackfand white pictures an "all or none typelight valve, as described,

above, the light valve is adjusted to give a f'ull:

exposure of the photographic paper'for current values between zero andsomevalue indicated at 88 which lies between the values 83 and 84. ,Forthe range of current values lying between 88 and 81 the light valveaperture is completely closed. Thus therewill be produced a blackandwhite picture which corresponds correctly to 3 the original, that is,the. light valve aperture will be closed when a current flowing throughthe string of the light valve has the amplitude 82 or the amplitude 83,thus reproducing white, and the aperture will be completely open whenthe energizing current for the lightvalve has the amplitude 84 or theamplitude 85, thus reproducing black. Thus a printed page consisting ofblack letters on a white ground, for example, would be correctlyreproduced and the background would not appear mottled.

What is claimed is: I 1. An electro-optical image producing syste forthe transmission of black and white pictures comprising means forscanning a field of view to produce an image current having components Iextending over arange of frequencies, means for suppressing certain lowfrequency components and transmitting the higher frequency compotendingover a range of values between a; mini' mumand a maximum value andnolight whenv the magnitude of the energizing current is out side ofsaidminimum and maximum values for controlling the production of an image ofsaid.

field, ofview and establishing-a uniformbackground under controlof saidtransmitted current. :4. An electro-optical {image producing. systemcomprising means for scanninga field ofview to produce an image currenthaving 'a direct component and alternating components extend-' ing overa range of frequencies, means for suppressing said directcomponent-andcertain low.

frequency alternating components and transmit.- ting the higher-frequenc'y components of said current-,means for producing a currenthaving a direct component and alternating components corresponding, tothe components of said transnents of said current, and light controlmeans which e'mits'the same amount of light when energized by current ofdifferent amplitudes for controlling the production of an image of saidfield of View and establishing a uniform background under control ofsaid transmitted current.

2. An electro-optical image producing system for the transmission ofblack and white pictures comprising means for scanning a field of viewto produce an image current having components ex tending over a range offrequencies, means for suppressing certain low frequency components andtransmitting the higher frequency components of said current, and lightcontrol means capable of emitting either a fixed amount of light or nolight only for controlling the production of an image of said field ofview and establishing a uniform background under control of saidtransmitted current.

-mittedi current, and light .control means which emits the same amountof-light when'energized. by current of different amplitudes-forcontrol-jling the production of an image of'said fieldeof view andj establishinga uniform background undercontrolOf said last mentioned-currenthavingdirect and alternatingcomponents.

5. A system for reproducing-.twortone pictures." which comprises atransmitting station and a receiving station, means at said transmittingstation for producing a beam of light, means for supporting a pictureand moving it with respect to said light beam so as to illuminateelemental areasthereof in succession, light sensitive elece tric meansfor receiving light reflected from said picture to produce a picturecurrent having a direct component and alternating components extendingover a range of frequencies, means for suppressing said ,directcomponent and certain low frequency alternating components andtransmitting the higher frequency components of said current to saidreceiving station, means for producing a light beam at the receiving station, means for supporting a picture surface and 3 moving it withrespect to saidlight beam, and

means responsive to the transmitted current for controlling the exposureof said-light sensitive surface to said light beam to reproducefthe picture, said means being adaptedto transmiteither a fixed amount of light.or no light only, to estab lish a uniform background.

6. A system for-producing images of a field of V view having only blackand white tone values, comprising a transmitting station and a'receivingstation, means at said transmitting station for scanning the field'ofview to set up an image current having components extending over. arange of frequencies, a transmission channel connecting said stationsfor transmitting the higher frequency components and suppressing thelower frequency components of said image current, and means for emittinga fixed amount of light or no light atsaid receiving station forproducing an image of said field of view and a uniform background undercontrol of said transmitted components.

7. 'In an image producing system asou'rceof image current representativeof an object having two light-tone values only and from which the lowfrequency components have been eliminated and which is per se in nomanner indicative. of the amplitudes of said eliminated components,means for receiving said image current and means for automaticallyproducing an image of the scanned object in two tones under control ofsaid received image current and independently of any other currentindicative of the amplitudes of said eliminated components 8. In animage producing system, a source 0 image current representative ofan'object" having two light-tone values only and from which the lowfrequency components have been eliminated-- andwhich is per se in nomanner indicative of the amplitudes of said eliminated components,

means for receiving said image current and means for automaticallyproducing an image of said tone values and to current amplitudes withina diflerent rangeto produce said other tone value. a

9. In an image producing system, a source of r of said received imagecurrent and independently 1 of any other current indicative-of theamplitudes" of said eliminated components, said meansin cluding amovable element the movement of" which is controlled by said receivedimagecurrent. f

10.In an image producing system, a source of image currentrepresentative of an object having two light-tone values only and fromwhich the low frequency components'have been eliminated and which isper'se in'no 'manner indicaimage current representative of' an objecthaving two light-tonevaluesxonly'and from which the low frequencycomponents have been" eliminated and which is per se in no manner indicative of the amplitudes of said eliminated components, means forreceiving said image current and means for automatically producing animage of the scanned object in two tones under control tive of theamplitudes of said eliminated components, means for receiving said imagecurrent and means for-automatically producing an image of the scannedobject in two tones under control of said receivedimage currentand'independently of any other current indicative of the amplitudes ofsaid eliminated components; said means including a movable element theposition of-which is controlled in accordance with the instantane ousamplitudesof said reeiv'ed'current, one of said tone valuesbeing'produced-wh'en said movable element occupies'one of a pluralityof"posi-- tions within a predetermined range and the other of said tonevalues being produced when said movable elementoccupies one of aplurality of positions within a different range.

. HERBERT VES.

